Recurrent homozygous damaging mutation in <i>TMX2</i>, encoding a protein disulfide isomerase, in four families with microlissencephaly.

Abstract

Protein disulfide isomerase (PDI) proteins are part of the thioredoxin protein superfamily. PDIs are involved in the formation and rearrangement of disulfide bonds between cysteine residues during protein folding in the endoplasmic reticulum and are implicated in stress response pathways. Eight children from four consanguineous families residing in distinct geographies within the Middle East and Central Asia were recruited for study. All probands showed structurally similar microcephaly with lissencephaly (microlissencephaly) brain malformations. DNA samples from each family underwent whole exome sequencing, assessment for repeat expansions and confirmatory segregation analysis. An identical homozygous variant in <i>TMX2</i> (c.500G>A), encoding thioredoxin-related transmembrane protein 2, segregated with disease in all four families. This variant changed the last coding base of exon 6, and impacted mRNA stability. All patients presented with microlissencephaly, global developmental delay, intellectual disability and epilepsy. While <i>TMX2</i> is an activator of cellular <i>C9ORF72</i> repeat expansion toxicity, patients showed no evidence of <i>C9ORF72</i> repeat expansions. The <i>TMX2</i> c.500G>A allele associates with recessive microlissencephaly, and patients show no evidence of <i>C9ORF72</i> expansions. <i>TMX2</i> is the first PDI implicated in a recessive disease, suggesting a protein isomerisation defect in microlissencephaly.